The present invention relates to a method and apparatus for radio transmission of digital information.
Methods and apparatus of this kind have already been implemented as radio LANs that utilize 2.4-MHz-band spread spectrum communication waves, but their transmission rates are of mostly 2 Mbps at most. Based not only on the recently growing CPU performance of terminal equipment but also on expectations for expanding practical applications containing pictorial images, graphics and similar image information, there is a strong demand for higher transmission rate.
To keep up with this current trend, IEEE 802.11 is now proceeding toward standardization of a new transmission system which achieves a maximum transmission rate as high as a bit rate of 11 Mbps, in addition to the existing standard system of the 2-Mpbs maximum transmission rate which uses the 2.4-MHz-band spread spectrum communication wave. The new transmission system is called a CCK system and is scheduled for practical utilization also in Japan in the near future.
Even if such a CCK system is put to practical use, however, the effective throughput is 5 to 6 Mbps at maximum and it is impossible to implement by radio LAN the same environment as the Ethernet of a 10-Mbps throughput.
Furthermore, a high-quality real-time image transmission system requires a throughput of above 6 Mbps, but the CCK system can barely satisfy this requirement and its transmission rate is far from sufficient.
The hidden factors that have hindered so far the implementation of satisfactory transmission rate despite the demand for higher transmission rate of the radio LAN are restrictions imposed on the radio frequency bandwidth by the rules of the Radio Law of Japan and a low frequency utilization factor of the spread spectrum system itself.
In Japan the radio frequency bandwidth allotted by the Radio Law to the spread spectrum communication system is 26 MHz in the 2.4 GHz band. On this account, the upper limit of the chip rate in a DS (Direct Sequence) system is 26 Mcps. The chip rate of 26 Mcps is, however, a theoretical upper limit value in the case of band limitation by an ideal Nyquist filter, and the upper limit of the actual chip rate is far lower.
That is, if a signal of a chip rate above 20 Mcps is band-limited by a Nyquist filter, the sampling frequency of a digital-to-analog converter becomes 40 MHz and a sharp band limitation of the converted signal is required; hence, in practice, no band limitation is involved, but instead band limitation of the base band is carried out by an analog filter after the digital-to-analog conversion. The upper limit of the chip rate in this instance is in the range of about 11 to 12 Mcps. The chip rate in IEEE 802.11 standard is of 11 Mcps and meets with this requirement.
When the chip rate is of 11 Mcps, the radio transmission rate without spread spectrum is 22 Mbps in case of QPSK (Quadrature Phase-Shift Keying) modulation. That is, when no spread spectrum takes place, the radio transmission rate is twice the chip rate.
On the other hand, the radio transmission rate for spread spectrum is of 22 Mbps in case of QPSK modulation according to the existing IEEE 802.11 standard and 11 Mbps according to a new standard planned to be work out in the near future. The transmission rates of these standard systems are of (1/5.5) and (1/2) timers higher than the chip rate, respectively.
The transmission of the IEEE 802.11 standard trades off the radio transmission rate for improvement of the multi-path resistance and the narrow-band disturbance resistance, and hence cannot achieve a sufficient transmission rate.
To implement the same environment as the Ethernet of the 10 Mbps transmission rate by the radio LAN, it is necessary, in general, to increase the radio transmission rate up to approximately 15 Mbps or more. Moreover, it is usually recommended that a radio transmission rate above 20 Mbps is needed to use an application that requires a large transmission capacity such as a real-time image transmission under the radio LAN environment.
Nevertheless, the radio transmission rate of the radio LAN now commercially available is on the order of 2 Mbps at the highest. Even if a radio LAN is commercially introduced in compliance with the afore-mentioned new IEEE standard intended for a maximum of 11 Mbps transmission rate, the above-mentioned requirement cannot be met.
An object of the present invention is to provide a radio transmission method and apparatus for digital information which offer a radio transmission rate twice or more higher than the chip rate and permit high-speed, high-quality transmission at a transmission rate of above 20 Mbps at a 26-MHz band in the 2.4-GHz band.
To attain the above objective, there is proposed a radio transmission method for digital information according to the present invention comprising:
from a transmitting side,
transmitting, over a radio transmission line, a modulated radio-frequency wave obtained by modulating a radio-frequency wave by a transmission baseband IQ signal, which is obtained by frame-synthesizing a baseband digital IQ signal modulated by the digital information to a spread spectrum IQ code sequence whose auto-correlation side lobe is equal to zero; and
on the receiving side,
separating a received signal of the spread spectrum code sequence and a received signal of the baseband digital-modulated IQ signal from a received baseband signal obtained by receiving the modulated radio-frequency wave, and
demodulating a waveform-equalized signal,which is obtained by waveform-equalizing of the received signal of the baseband digital-modulated IQ signal by the use of transmission parameters of the radio transmission line derived from a despread spectrum signal obtained by despreading the spread spectrum IQ code sequence, to obtain a received data signal of the digital information transmitted from the transmitting side.
At the receiving side: an impulse response of the radio transmission line is derived from an average signal of the despread spectrum signal;
a pilot-synchronous detection signal is calculated which is obtained by multiplying each modulated IQ signal vector in the received signal of the baseband digital modulation signal by a complex conjugate of the impulse response for a first incoming wave;
inter-symbol interference is calculated by summing up the products of the respective signal vectors corresponding to received data signal of the baseband digital-modulated IQ signal and the impulse response; and
the inter-symbol interference is subtracted from the pilot-synchronous detection signal, by which the waveform equalization can be performed.
There is further proposed a radio transmission apparatus for digital information according to the present invention,comprising a transmitter and a receiver;
the transmitter comprising:
preamble generating means for outputting spread spectrum IQ code words, as a preamble signal, whose auto-correlation side lobe is equal to zero during a preamble period;
information signal modulating means for outputting a modulated IQ signal representative of transmission data;
frame synthesizing means for synthesizing the preamble signal to the modulated IQ signal to obtain a baseband digital-modulated IQ signal;
up-converting means for up-converting the baseband digital-modulated IQ signal to provide an up-converted communication wave; and
a transmitting antenna for transmitting said up-converted communication wave; and
the receiver comprising:
a receiving antenna for receiving the communication wave transmitted;
down-converting means for down-converting the communication wave signal received by the receiving antenna for conversion to a baseband digital-modulated IQ signal;
analog-to-digital converting means for converting the baseband digital-modulated IQ signal to a digital signal;
matched filter means for deriving a despread spectrum signal from the digital baseband digital-modulated IQ signal provided from the analog-to-digital converting means;
transmission parameter estimation means for deriving transmission parameter information necessary for waveform-equalization from the despread spectrum signal provided from the matched filter means;
waveform-equalizing means for equalizing the waveform of the digital-modulated IQ signal on the basis of the transmission parameter information; and
demodulating means for demodulating a waveform-equalizing signal provided from the waveform-equalizing means.